The present invention relates to the field of sulphonic acids and has more particularly as subject-matter the purification of alkanesulphonic acids, such as methanesulphonic acid (MSA), for the purpose of reducing the content of sulphuric acid therein.
Alkanesulphonic acids and more particularly MSA are of use as esterification catalysts and in the plating of conductive metals.
The synthesis of these acids by oxidation of the corresponding thiol, by hydrolysis of an alkanesulphonyl halide or by oxidation of dimethyl disulphide generates various impurities, the presence of which can prove to be a nuisance during use. This is particularly the case for sulphuric acid, which can be found in MSA at concentrations ranging from a few hundred to a few thousand ppm and whose presence is harmful in the use of MSA in the plating of conductive metals; for this application, the commercial specification requires a sulphuric acid content of less than 150 ppm.
In contrast to other impurities, such as hydrochloric acid, sulphuric acid cannot be removed by stripping. Various methods for removing sulphuric acid are known but none of these methods, whether physical (fractional crystallization or separation through a nanofiltration membrane) or chemical (precipitation of alkaline earth metal sulphates, selective electrochemical reduction of H2SO4 or selective reduction of H2SO4 by hydrogen sulphide) gives a satisfactory result.
It has now been found that the content of sulphuric acid in an alkanesulphonic acid can be greatly reduced by bringing the latter into contact with a basic anion-exchange resin.
A person skilled in the art knows that basic resins, in particular strong basic resins, are capable of attaching the various anions with a variable affinity which depends on the nature of the anion under consideration and on the potential number of charges capable of being carried by this anion.
It is, on the other hand, surprising for a basic anion-exchange resin to be able to selectively attach sulphuric acid present at low concentration in a highly concentrated alkanesulphonic acid (for example MSA). This is because it might have been expected that a high concentration of alkanesulphonic acid would prevent any selectivity from being expressed, all the more so since, in this highly acidic medium, the sulphuric acid is probably not ionized and must therefore be exchanged with the counterion present on the resin under conditions which are a priori not very favourable.
A subject-matter of the invention is therefore a process for the purification of an alkanesulphonic acid in order to reduce the content of sulphuric acid therein and, incidentally, that of anions other than the alkanesulphonate anion, characterized in that it comprises at least one stage in which an aqueous solution of the alkanesulphonic acid to be purified is brought into contact with a basic anion-exchange resin.
Although the process according to the invention is, in the first place, targeted at the purification of MSA, it can also be applied to that of any water-miscible alkanesulphonic acid, in particular acids comprising up to 12 carbon atoms and more particularly those comprising up to 4 carbon atoms, such as ethanesulphonic acid, n-propanesulphonic acid and n-butanesulphonic acid.
The content by weight of alkanesulphonic acid in the aqueous solution of alkanesulphonic acid to be purified can vary within wide limits (10 to 90%, depending on the nature of the acid) but it is advantageously between 60 and 80% and generally in the region of 70% (usual content of commercial MSA. solutions).
The basic anion-exchange resins to be used in the implementation of the process according to the invention are well known and are commercially available. Use may be made of weak basic resins, such as those carrying secondary amine (for example dialkylamino) functional groups, but it is preferable to employ strong basic resins, such as those carrying quaternary ammonium functional groups, in particular xe2x80x94N⊕R3 groups where R is a C1 to C4 alkyl radical, preferably methyl. These functional groups are generally attached to a polystyrene-divinylbenzene copolymer with a macroporous structure. The preferred anion-exchange resins according to the invention are those sold under the name Diaion(copyright) HPA25 by the Company Resindion and under the name Amberlite(copyright) IRA92 by the Company Rohm and Haas.
These resins are generally not very stable thermally. For this reason, they must be brought into contact with the aqueous solution of alkanesulphonic acid to be purified at a temperature not exceeding that resulting in decomposition of the resin- The latter is generally below 120xc2x0 C.; consequently, the process according to the invention is advantageously carried out below 80xc2x0 C. and preferably at room temperature.
It is preferable to use resins in their chloride form as, after bringing into contact with the alkanesulphonic acid to be purified and separating by any appropriate known means (in particular by filtration, percolation, centrifuging, and the like), the hydrochloric acid formed by the exchange of the sulphate and chloride anions can be easily removed by stripping the purified solution. This operation can be carried out under vacuum and under warm conditions (25 to 120xc2x0 C., preferably from 30 to 80xc2x0 C.), optionally with addition of steam. In the case of MSA, it is thus possible to obtain an acid comprising less than 100 ppm of sulphuric acid and less than 10 ppm of hydrochloric acid.
The efficiency of the purification obviously depends on the time during which the aqueous solution of alkanesulphonic acid is brought into contact with the basic resin and on the state of saturation of the latter. The throughput (bv), which is the volume of liquid to be treated with respect to the volume of resin and per hour, can range from 0.1 to 5 but it is advantageous to carry out the purification at a value bv of less than 2.5 and preferably at most equal to 0.5.
In the following examples, which illustrate the invention without limiting it, the percentages and ppm are expressed by weight. The experimental device used was composed of a peristaltic pump, feeding, from a reservoir of MSA to be purified, a glass column comprising 20 ml of anion-exchange resin, and, at the outlet of this column, of a receptacle for the purified MSA.